We thank Madam Grace Chua Siew Hwee for her feedback (Concerns about thermal fogging on human health; Forum Online, March 4).
The National Environment Agency (NEA) emphasises source reduction of the Aedes mosquito as the primary focus of our dengue control efforts.
In dengue cluster areas, the NEA carries out vector control operations to remove mosquito breeding habitats, and conducts indoor misting of insecticides and outdoor fogging to kill adult mosquitoes, and oiling of stagnant water (for example, in drains) to kill mosquito larvae.
In such areas with active transmission, outdoor fogging and indoor misting are necessary because there may be infected adult mosquitoes in both outdoor and indoor areas that need to be destroyed.
These methods are, however, effective only if the chemical has direct contact with the mosquitoes, and thus have to be repeated frequently, as new batches of mosquitoes will continue to emerge until all breeding habitats are found and removed. Fogging should also be carried out in the earlier part of the morning or later part of the afternoon, when Aedes mosquitoes are known to be most active.
Given these considerations, routine fogging is not a sustainable vector control measure - source reduction is still a more effective and sustainable strategy. This is why we encourage all stakeholders to check their premises regularly for potential breeding habitats.
The active ingredient of the insecticide used for fogging does not have a long residual effect on the environment, and the concentration used is low and deemed safe by the World Health Organisation.
When indoor misting has to be conducted, infants, the elderly, pregnant women and those with respiratory problems are asked to leave the premises before the commencement of indoor misting and allowed to re-enter only 10 minutes after misting has been completed.
Residents will also be asked to keep their pets outside the premises and cover their aquariums, and cover food and utensils which may be exposed during fogging or misting operations.
During outdoor fogging or misting, residents are advised to close their windows, remain indoors and cover their food and utensils.
Residents will be informed in advance via notices or by the pest control operator when the use of fogging and misting is necessary.
Director, Environmental Public Health Operations
National Environment Agency